Restoring ? cell mass in patients with diabetes would provide the blood glucose regulation necessary to greatly reduce their risk of developing long-term complications such as heart disease, blindness, kidney disease and neuropathy. However, ? cell regeneration is limited and new strategies are needed to promote this process. While studying the molecular events involved in pancreatic islet vascularization, our laboratory discovered a new model of ? cell proliferation and islet regeneration. This proposal seeks to understand the processes involved in promoting this regenerative response. Transient overall expression of vascular endothelial growth factor-A (VEGF-A) in ? cells caused intra-islet endothelial cell proliferation, accompanied by ? cell loss. Remarkably, withdrawal of the VEGF-A stimulus is followed by ? cell proliferation and islet regeneration. This regenerative response is dependent on the local islet microenvironment, which consists of endocrine cells, endothelial cells, and recruited circulating bone marrow-derived cells (RCBCs) that infiltrate islets upon VEGF- A induction and are required for ? cell proliferation. The identity of these cells, and the mechanism by which they infiltrate islets and stimulate ? cell proliferation, are unknown. Using a next-generation sequencing approach, we found that chemokine receptor 4 (CXCR4) expression increases in islets upon RCBC infiltration. Studies in other tissues demonstrated that VEGF-A expression can lead to infiltration of circulating bone marrow-derived myeloid cells, mediated by CXCR4 and its obligate ligand, stromal-cell derived factor-1 (SDF- 1). We hypothesize that a specific subset of bone marrow-derived cells is capable of inducing ? cell proliferation, and that the infiltration of these cells is dependent on CXCR4-SDF-1 interactions. To test these hypotheses, we propose to first define the subset of bone marrow-derived cells capable of inducing cell proliferation in islets by identifying the specific profile of cell surfce markers expressed on RCBCs isolated from infiltrated islets. Then we will ascertain whether bone marrow cells with this same profile are capable of inducing ? cell proliferation following VEGF-A induction in islets transplanted into myeloid-depleted mice. Secondly, we will determine whether infiltration of bone marrow-derived cells into islets in response to VEGF-A is dependent on the CXCR4-SDF-1 receptor-ligand interaction by testing whether isolated RCBCs express CXCR4 and migrate in response to SDF-1, then blocking CXCR4 in mice and evaluating islets for RCBC infiltration and ? cell proliferation. These proposed studies will clearly define a distint subset of bone marrow- derived cells capable of inducing ? cell proliferation, and determine how to promote their recruitment to pancreatic islets, which will allow us to further define the mechanism by which these cells promote ? cell proliferation and to develop new therapies for diabetes.